1. Field of Invention
The invention relates to a device for measuring the microwave power inside a waveguide. More particularly, the invention discloses the structure of a micro strip directional coupler for measuring microwave power in different directions inside a waveguide.
2. Related Art
The microwave power needed in a typical microwave heating system is usually hundreds of watts. If one uses a normal microwave power meter to measure the microwave source, the large power output may easily damage the power meter. Therefore, a microwave coupler is often used in this case. The hundreds of watts of microwave power output is coupled with the microwave coupler in such a way that the measured power is only of mW order. The signal coupled by the microwave coupler can be directly read out by microwave power meters. After a systematic conversion, one can obtain the output power of the microwave source.
The microwave heating system is mainly comprised of a power supply, a magnetro, a waveguide, a controller and a cavity. The magnetro is one type of microwave generators. It provides such information as temperature, humidity and weight through detectors, and its output is controlled by a time switch or a feedback controller. The waveguide specifically refers to all kinds of hollow metal waveguides and surface-wave waveguides. Taking a microwave oven as an example, the energy of the microwave generated by the magnetro is transferred in the form of waves through a hollow metal tube to a heating cavity. The microwave coupler is a measuring device for measuring the microwave power.
There are many known microwave couplers. They are consisted of appropriate coupling structures between a main transmission line and an auxiliary transmission line. The transmission line that a directional coupler uses to transmit microwave coupling signals can be a coaxial line, a strip line, a micro strip line, a metal waveguide, or a medium waveguide. The coupling structure can be a coupling hole, a coupling branch line, and a continuous structural coupling.
The techniques disclosed in the U.S. Pat. Nos. 4,297,658, 4,792,770, 5,043,684, and 5,185,046 are mainly waveguide directional couplers that utilize two parallel waveguides. The coupling structure is achieved using several coupling holes or coupling windows or slits. The directional coupler using a metal waveguide often has a high conductivity (e.g. copper, aluminum or stainless steel) in order to minimize the energy loss during the transmission process. Furthermore, the inner walls are as smooth as possible and the metal connecting places are made as few as possible. The cross section of the waveguide can be rectangular or circular. They differ in microwave transmission effects, structural designs, and properties of objects to be heated. They often require more delicate machining and are more difficult in manufacturing.
The technique disclosed in the U.S. Pat. No. 3,721,921 is a directional coupler using a coupling branch structure. Conventional directional couplers that use strip lines or micro strip lines are of two types. One is a rotational design. It rotates the direction ,of a transmission line so that the transmission line and the coupling hole on the waveguide reach an optimal relative position for coupling. The other is a fixed design. Once the coupler and the waveguide are combined and fixed, the relative position and angle between the transmission line and the coupling hole on the waveguide are unchangeable. The drawback of the rotation-type coupler is in that the structure may become loose and affect the coupling effect.
An objective of the invention is to provide a new, simple structure of a directional coupler for microwave coupling cavities that is easy in manufacturing.
The manufacturing process of the disclosed directional coupler only involves the steps of making print circuit boards, assembly and electroplating. The step of making print circuit boards is to make the first carrier and the second carrier that contain micro strip lines. The first carrier is implemented by forming a plane copper foil on a fiber substrate. The assembly is to combine the first carrier, the second carrier, and two signal connectors together. The electroplating step covers the surfaces other than the two signal connectors and the position reserved for a coupling hole on the first carrier by a conductive metal (e.g. copper or gold), forming a metal shell.
The micro strip type directional coupler of the invention does not use a hollow metal waveguide structure. Only connectors such as screws are needed to fix the directional coupler on an outer side of the coupling hole of the waveguide. Therefore, the invention is easy to make and install and does not occupy too much space.